Abstract: Mordenite type zeolitic alteration was identified in the Upper Cretaceous andesitic, dasitic rocks in Şile region, istanbul. Silisification (quartz and opal-CT) and low degree clay alteration (smectite and illite) were recognized in mordenite - rich samples, whilehigh degree clay alteration was determined in the samples which do not contain any mordenite mineralization. These alteration products have been developed in glassy components and also fill cavities and fissures of volcanic rocks. Besides, carbonate, chloriteand kaolinite type alterations were also identified in lioles and in pltenocryls. Mordenite crystals display thick tabular or prismaticmorplwlogy, 40x80 jum long. Hexagonal cross sections of the crystals have thicknesses changing in orders of 10 - 40 jU/n. The average chemical composition of the mordenite crystals can be expressed by the chemical formula of"Ca4j (Al!3Si36096). 11 H2O". DTAcurves of mordenites show endotherms at 110 XJ and 560 C According to the TGA curves of mordenitesthe total amount of weightloss below 950 V is about 11%.